In a typical swash-plate type variable capacity compressor, the tilt angle of a swash-plate can be controlled by the change in the pressure of the refrigerant which is controlled by the pressure control valve depending on the thermal load. By controlling the tilt angle of a swash-plate, the stroke distance of the piston is changed thereby changing the discharge capacity of refrigerant and thus controlling the temperature of an evaporator.
The pressure control valve can be either internal-controlling type or external-controlling type, and a typical structure of swash-plate type variable capacity compressor is disclosed in Japanese laid open patent application No. 2001-107854.
In vehicles having an air conditioner equipped with the swash-plate type variable capacity compressor as described above, the air conditioner should operate properly at the initial stage of the operation depending on the deviation between the target evaporator temperature and an actual evaporator temperature. For example, the discharge capacity of the compressor should be controlled so that the evaporator temperature reaches the target temperature, to keep passengers from being uncomfortable and prevent the occurrence of loud noise.
Japanese laid open patent application No. 2003-200730 discloses a method of controlling an air conditioner wherein proportional-integral control is carried out on the discharge capacity of the compressor by setting the control value (duty) of the pressure control valve depending on the deviation between the target evaporator temperature and actual evaporator temperature.
This technique has an advantage of making evaporator temperature drop quickly. However, when the initial value of the pressure control valve and the actual evaporator temperature are higher than the target temperature, considering the actual evaporator temperature is high at the initial stage of operation, the controlled current value of the pressure control valve should be further increased in order to fully decrease the actual evaporator temperature. Thus, excessive undershoot of the evaporator temperature may occur and it takes a considerable time for the current value to decrease and converge to stabilization.
Japanese laid open patent application No. 2002-327686 discloses a method in which the capacity of the pressure control valve becomes increasingly larger from its minimum value during operation of the air conditioner.
In this method, the discharge capacity of the compressor is slowly increased from its minimum value, preventing operational shock and making passenger comfortable and improving the problem of noise. This method, however, has a problem of taking a long time for stabilization to reach the target evaporator temperature.
Also, the prior art methods described above can cause a problem of deteriorating convergence to reach the target evaporator temperature and thus cause instability since the capacity of the pressure control valve is fixedly controlled regardless of the variable need for high thermal load control or low thermal load control on the compressor.